Walk in

: : : I'm translating a book in serbian language and I desperately need the meaning of the phrase
: : : WALK IN SHOE LEATHER

: : I haven't heard that. Seems like I've heard something like, "The meanest man in shoe leather." Most men wear shoes of some sort so this man is the meanest of that large category.

: ESC, your phrase is an updating of a Shakespeare quote: "as proper a man as ever trod upon neat's leather" [Julius Caesar, Scene 1] ("Neat" is an old word for "cow".) As you say, the phrase is an elaborate way of saying "as [mean/ proper/ whatever you like]as ever was".

there may be some ambiguity as to what you want - "to walk a mile in someone's shoes" is a shortened version of the expression that you should not condemn somebody until you have walked a mile in their shoes - i.e. you should not usually condemn without understanding somebody's experience.

to be in somebody's shoes - means to be in their position - as in "I wouldn't want to be in your shoes when your wife finds out!" - it is often said jokily.

Perhaps if it were a clothing catalogue the meaning might be to 'walk-in' shoe leather - as new shoes need to be worn to mould them better to the feet of the wearer - this is called 'walking in' and is particularly used about boots - particularly hiking/walking boots.